Adding auto-completion

[TylerGrantSmith]

I'm not sure if this is something you'd be interested in adding, but I've found it useful for my own purposes to implement tab-completion in Rstudio for my targets objects. This proof-of-concept implementation works with tar_read and tar_load using code adapted from paletteer (https://github.com/EmilHvitfeldt/paletteer/blob/master/R/completeme.R). It works great!

Example of what it looks like:

[wlandau]

Nice idea! I was wondering how autocompletion could be done. Would be nice to have autocompletion for the arguments of tar_load() and tar_read(). However, I am concerned about using ideas we get from reading GPL code. Both palenteer and completeme are GPL-3, so targets would need to switch from MIT to GPL-3, which is too high a cost. Also, I think both targets and the community would be better served with fully general-purpose function argument autocompletion functionality on CRAN. (Unfortunately, I see the last commit to completeme was April 2019, so maybe it would need to be revived or superseded somehow.)